The application generally relates to a capacity control system and method for a multi-stage centrifugal compressor. The application relates more specifically to systems and methods for managing the control of a variable geometry diffuser or pre-rotation vanes for each compressor in a multi-stage centrifugal compressor to optimize compressor operating efficiency while avoiding surge conditions in the compressor.
Design conditions for a centrifugal compressor may be defined by the gas flow, temperature and pressure conditions at suction and discharge. The compressor may operate continuously at conditions that are close to the design conditions, or the operating environment may deviate widely from design conditions during extended periods of time. Compressors used in HVAC systems may be subject to wide variations. The gas flow depends on the demand for cooling load, while the pressure conditions, especially the condensing pressure, depend on the ambient temperature conditions.
When operating conditions deviate from the design conditions a centrifugal compressor may encounter instabilities such as surge or stall during operation. Surge or surging is a transient phenomenon having oscillations in pressures and flow, and can result in complete flow reversal through the compressor. When surging, a compressor may be unable to deliver the desired flow at the desired pressure conditions. Furthermore, surging can cause excessive vibrations in both the rotating and stationary components of the compressor, and may result in compressor damage. Various devices and control parameters can be used to adjust the compressor operation to desired flow and pressure conditions while avoiding compressor surging. The simplest way to reduce the flow of a centrifugal compressor is to reduce its speed. Turbine drives, for example, may be operated at variable speed, or electric motors with electrical power supplied through a variable speed drive (VSD). When available, speed reduction can be used only to a limited extent to avoid surge. When speed reduction is not possible, another solution is to use a flow reduction device (“FRD”), such as pre-rotation vanes (“PRV”) at compressor inlet, or a variable geometry diffuser (“VGD”) to reduce the flow of the compressor. When the possibilities of reduced speed and of various FRD's have been exhausted, another technique to correct a surge condition involves the opening of a hot gas bypass valve to return some of the discharge gas of the compressor to the compressor inlet to increase the flow at the compressor inlet. Depending on their availability on the machine, the settings of aforementioned devices, namely the variable speed drive, pre-rotation vanes, variable geometry diffuser and hot gas by-pass are managed by a stability control algorithm intended to keep the machine in stable operation out of surge at the desired operating conditions, while optimizing its efficiency.
Active magnetic technology in the form of electromagnetic bearings is currently utilized in some turbomachinery drivelines, such as motors, compressors or turbines, to reduce friction while permitting free rotational movement by levitating rotors and shafts during operation. Electromagnetic bearings replace conventional technologies like rolling element bearings or fluid film bearings in the operation of such rotating apparatus, but require centering of the shaft within the electromagnetic bearings. When the compressor is operating normally, there is no mechanical contact between the rotating shaft and the stationary parts of the driveline. In the event of an unusual overload conditions such as surge in a turbo machine the load capacity of the bearings can be exceeded; the compressor shaft can no longer be supported by the electromagnetic bearings, resulting in a safety trip of the magnetic bearings.
In HVAC systems including a variable speed motor, the stability control algorithms are used in conjunction with the variable speed drive. Adaptive capacity control logic utilizing system operating parameters and compressor FRD position information can be used, e.g., to operate the compressor at a faster speed when a surge is detected while stability control algorithms are in a surge reacting state. Past performance parameters can be mapped and stored in memory to avoid future surge conditions by the adaptive capacity control logic. A description of an exemplary adaptive capacity control process is provided in U.S. Pat. No. 4,608,833 which patent is hereby incorporated by reference.
However, where magnetic bearings are utilized in the compressor, an adaptive control logic that relies on the compressor entering a surge condition is undesirable, to the extent that a surge condition poses an increased risk of a system shutdown that causes nuisance trips and may reduce the life time of the bearings.
Intended advantages of the disclosed systems and/or methods satisfy one or more of these needs or provide other advantageous features. Other features and advantages will be made apparent from the present specification. The teachings disclosed extend to those embodiments that fall within the scope of the claims, regardless of whether they accomplish one or more of the aforementioned needs.